This invention relates to the recovery of marketable products such as oil and gas from hydrocarbon bearing deposits such as oil shale or tar sand by the application of electromagnetic energy to heat the deposits. More specifically, the invention relates to a method and system including use of a high power radio frequency signal generator and an arrangement of elongated electrodes inserted in the earth formations for applying electromagnetic energy to provide controlled heating of the formations. Still more particularly, the invention relates to such method and system wherein reactive elements are disposed along respective elongated electrodes to provide predetermined characteristics for the transmission line formed thereby so as to develop a predetermined heating pattern
Materials such as oil shale, tar sands, and coal are amenable to heat processing to produce gases and hydrocarbonaceous liquids Generally, the heat develops the porosity, permeability and/or mobility necessary for recovery. Oil shale is a sedimentary rock which, upon pyrolysis or distillation, yields a condensable liquid, referred to as shale oil, and noncondensable gaseous hydrocarbons. The condensable liquid may be refined into products which resemble petroleum products. Tar sand is an erratic mixture of sand, water and bitumen with the bitumen typically present as a film around water-enveloped sand particles. Using various types of heat processing, the bitumen can be separated. Also, as is well known, coal gas and other useful products can be obtained from coal using heat processing.
In the destructive distillation of oil shale or other solid or semisolid hydrocarbonaceous materials, the solid material is heated to an appropriate temperature and the emitted products are recovered. The desired organic constituent of oil shale, known as kerogen, constitutes a relatively small percentage of the bulk shale material, so very large volumes of shale need to be heated to elevated temperatures in order to yield relatively small amounts of useful end products. The handling of the large amounts of material is, in itself, a problem, as is the disposal of wastes. Also, substantial energy is needed to heat the shale, and the efficiency of the heating process and the need for relatively uniform and rapid heating have been limiting factors on success. In the case of tar sands, the volume of material to be handled, as compared to the amount of recovered product, is again relatively large, since bitumen typically constitutes only about ten percent of the total, by weight. Material handling of tar sands is particularly difficult even under the best of conditions, and the problems of waste disposal are, of course, present there, too.
A number of proposals have been made for in situ methods of processing hydrocarbonaceous deposits and recovering valuable products therefrom. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the formation. Valuable constituents of the formation, including heated liquids of reduced viscosity, may be drawn to the surface by a pumping system or forced to the surface by injecting another substance into the formation. It is important to the success of such methods that the amount of energy required to effect the extraction be minimized.
It has been known to heat relatively large volumes of hydrocarbonaceous formations in situ using radio frequency energy. This is disclosed in Bridges and Taflove U.S. Pat. No. Re. 30,738. That patent discloses a system and method for in situ heat processing of hydrocarbonaceous earth formations wherein a plurality of conductive means are inserted in the formations and bound a particular volume of the formations. As used therein, the term "bounding a particular volume" was intended to mean that the volume was enclosed on at least two sides thereof In the most practical implementations, the enclosed sides were enclosed in an electrical sense, and the conductors forming a particular side could be an array of spaced conductors. Electrical excitation means were provided for establishing alternating electric fields in the volume. The frequency of the excitation means was selected as a function of the dimensions of the bounded volume so as to establish a substantially nonradiating electric field which was substantially confined in said volume. In this manner, volumetric dielectric heating of the formations occurred to effect approximately uniform heating of the volume.
In the preferred embodiment of the system described in that patent, the frequency of the excitation was in the radio frequency range and had a frequency between about 1 MHz and 40 MHz. In that embodiment, the conductive means comprised conductors disposed in respective opposing spaced rows of boreholes in the formations. One structure employed three spaced rows of conductors which formed a triplate-type of transmission line with the formations as the dielectric between conductors. Particularly as the energy was coupled to the formations (dielectric) from electric fields created between respective conductors, such conductors were, and are, often referred to as electrodes.
The reissue patent disclosed the imposition of standing electromagnetic waves on the electrodes embedded in the formations. Such standing waves create a sinusoidally varying electric field along the length of the transmission line formed by the electrodes, with peaks and nodes separated by a distance equal to one quarter of the wavelength (.lambda./4) of the signal applied to the electrodes. This, in turn, creates a heating power which varies in strength along the length of the electrodes and which, consequently, gives rise to heating and temperature variations along the length of the electrodes. As it was desired to provide relatively uniform heating, the system disclosed in that patent provided compensation for such variations in the following ways: (1) by modifying the phase or frequency of the excitation signal, and (2) by decreasing the effective insertion depth of some of the conductors either by pulling some of the conductors part way out of the formation or by employing small explosive charges to sever end segments of the conductors. In addition, as was stated at column 12, lines 43 to 62, capacitive loading could be employed to minimize standing wave amplitude reduction effects, as, for example, by inserting capacitors at regular intervals along the central electrodes for partially canceling the effective series inductance of the center conductors.
In copending application Ser. No. 343,903, filed Jan. 29, 1982 by Bridges and Taflove for Apparatus and Method for In Situ Controlled ,Heat Processing of Hydrocarbonaceous Formations now U.S. Pat. No. 4,449,585, issued May 22, 1984, and commonly assigned, there was disclosed an improvement on the method and system of the reissue patent wherein power was applied at one end of the transmission line and the termination of the distal end of the line was controlled. Terminating one end of the structure with different impedances at different times produced electric field standing waves of different respective phase at that end at the selected frequency. In certain embodiments the difference in phase of the standing waves was made substantially 90.degree. in order that the resultant heating effects for the two respective standing waves be 180.degree. out of phase. At least where the dielectric properties of the formations were relatively uniform, the combined effect of such change of phase was thus to provide substantially uniform heating when the product of the amplitude-squared of the electric field standing wave and the dwell time in the respective phase was substantially the same in the two modes. Such 90.degree. phase shift might be effected by terminating the line alternately with substantially effectively open and short circuits. Pure resistive and pure reactive loads and combination resistive and reactive loads might also be used.
The copending application Ser. No. 343,903 also contemplated a number of desired controlled heating patterns in addition to uniform. These were achieved by utilizing different dwell times and/or different amplitudes of electric field for the different respective standing wave patterns. The use of different frequencies provided further flexibility in the heating patterns that could be established, particularly where the line was terminated differently at the respective frequencies. Also contemplated was the application of electromagnetic energy at different frequencies at the same time while terminating the line differently at the different frequencies to provide a particular programmed heating pattern.